The present invention relates generally to drop-on-demand liquid emission devices such as, for example, ink jet printers, and more particularly such devices which employ an electrostatic actuator for driving liquid from the device.
Drop-on-demand liquid emission devices with electrostatic actuators are known for ink printing systems. U.S. Pat. No. 5,644,341 and U.S. Pat. No. 5,668,579, which issued to Fujii et al. on Jul. 1, 1997 and Sep. 16, 1997, respectively, disclose such devices having electrostatic actuators composed of a diaphragm and opposed electrode. The diaphragm is distorted by application of a first voltage to the electrode. Relaxation of the diaphragm expels an ink droplet from the device. Other devices that operate on the principle of electrostatic attraction are disclosed in U.S. Pat. No. 5,739,831, U.S. Pat. No. 6,127,198, and U.S. Pat. No. 6,318,841; and in U.S. Pub. No. 2001/0023523.
U.S. Pat. No. 6,345,884, teaches a device having an electrostatically deformable membrane with an ink refill hole in the membrane. An electric field applied across the ink deflects the membrane and expels an ink drop. This device is simple to make, but requires a field across the ink and is therefore limited as to the type of ink usable therewith.
IEEE Conference Proceeding xe2x80x9cMEMS 1998,xe2x80x9d held Jan. 25-29, 2002 in Heidelberg, Germany, entitled xe2x80x9cA Low Power, Small, Electrostatically-Driven Commercial Inkjet Headxe2x80x9d by S. Darmisuki, et al., discloses a head made by anodically bonding three substrates, two of glass and one of silicon, to form an ink ejector. Drops from an ink cavity are expelled through an orifice in the top glass plate when a membrane formed in the silicon substrate is first pulled down to contact a conductor on the lower glass plate and subsequently released. There is no electric field in the ink. The device occupies a large area and is expensive to manufacture.
U.S. Pat. No. 6,357,865 by J. Kubby et al. teaches a surface micro-machined drop ejector made with deposited polysilicon layers. Drops from an ink cavity are expelled through an orifice in an upper polysilicon layer when a lower polysilicon layer is first pulled down to contact a conductor and is subsequently released. There is no electric field in the ink. However, the device requires a high voltage for efficient operation and materials with special elastic moduli are required for manufacture.
The gap between the diaphragm and its opposed electrode must be sufficiently large to allow for the diaphragm to move far enough to alter the liquid chamber volume by a significant amount. Large gaps require large voltages to move the diaphragm, and large voltages require expensive circuitry and add to the assembly process. If the gap is made very small, the motion of the diaphragm is constrained and the area of the device must be made large.
In devices that rely on the elastic memory of the diaphragm to expel liquid drops, the diaphragm must return to its initial position under the force of its own tension and sheer stiffness. This is not always sufficient to overcome stiction; nor is tension and stiffness identical for each membrane.
When the diaphragm is distorted by application of a voltage to the electrode, the diaphragm has a tendency to snap all the way into contact with an underlying substrate as the diaphragm approaches the substrate. This generally occurs during the final third the diaphragm""s travel. This part of the motion is not under control.
According to a feature of the present invention, a drop-on-demand liquid emission device, such as for example an ink jet printer, includes an electrostatic drop ejection mechanism that employs an electric field for driving liquid from a chamber in the device. Structurally coupled, separately addressable first and second dual electrodes are movable in a first direction to draw liquid into the chamber and in a second direction to emit a liquid drop from the chamber. A third electrode between the dual electrodes has opposed surfaces respectively facing each of said first and second electrodes at an angle of contact whereby movement of the dual electrodes in the first direction progressively increases contact between the first and third electrodes, and movement of the dual electrodes in the second direction progressively increases contact between the second and third electrodes.